1. Field of the Invention
The present invention relates to a power steering system that can apply a steering force and a steering assistance force by operating an oil pressure cylinder in accordance with an input torque outputted from a steering input mechanism such as a steering wheel of a vehicle.
2. Description of the Related Art
A conventionally known power steering system of this kind is disclosed in Japanese Patent Unexamined Publication No. Showa 59 (1984)-118569 (=JP59-118569).
The above power steering system (referred to as “power steering gear” in its English abstract) includes:
1) a steering shaft mounted to a steering wheel,
2) an output shaft connected to a lower end of the steering shaft,
3) a pinion mounted to a lower end of the output shaft, and a rack meshed with the pinion,
4) an oil pressure cylinder which is an operation oil actuator connected to the rack,
5) a reversible oil pump supplying an operation oil pressure by way of a first oil passage and a second oil passage respectively to a first oil pressure chamber (right) and a second oil pressure chamber (left) which are pairwise and partitioned by a piston of the oil pressure cylinder,
6) an electromagnetic switch valve fitted to a bypass passage connected between the first oil passage and the second oil passage, the electromagnetic switch valve opening and closing the bypass passage.
In the traveling of a vehicle, ordinarily turning the steering wheel clockwise and counterclockwise may allow the following operations:
1) A torque sensor (after detecting a steering torque) may output a passage-closing signal to the electromagnetic switch valve by way of an electric controller.
2) The reversible oil pump may turn forward or reverse, thus supplying the operation oil pressure in the following manner:                i) Operation oil pressure in one of the first oil pressure chamber (right) and the second oil pressure chamber (left) is supplied to the other of the first oil pressure chamber (right) and the second oil pressure chamber (left).        ii) Operation oil pressure in one of the first oil passage and the second oil passage is supplied to the other of the first oil passage and the second oil passage.        
The operation oil sufficiently supplied to substantially an entire closed circuit including the other of the first oil pressure chamber (right) and the second oil pressure chamber (left) and the other of the first oil passage and the second oil passage may be pressurized by means of the reversible oil pump, thus expanding the other of the first oil passage and the second oil passage or smashing bubble in the operation oil, resulting in an apparent increase in capacity of the other of the first oil passage and the second oil passage. For compensating for the increase in capacity, a reservoir is used for sucking the operation oil by way of a check valve into the other of the first oil passage and the second oil passage for pressurization by means of the reversible oil pump, thus giving the steering assistance force.
Under the Japanese Patent Unexamined Publication No. Showa 59 (1984)-118569 (=JP59-118569) in which the apparent increase in capacity of the other of the first oil passage and the second oil passage is compensated for by the operation oil from the reservoir.
The above compensation may, however, cause variation in operation oil quantity between the first oil passage and the second oil passage. Thereby, changing the turning of the reversible oil pump from forward to reverse such that the operation oil pressure can be supplied to the one of the first oil passage and the second oil passage may cause a remaining pressure due to a speed reduction of the oil pressure of the other of the first oil passage and the second oil passage. Hereinabove, the speed reduction is attributable to the increased operation oil quantity (namely, an increase equivalent to the compensation) of the other of the first oil passage and the second oil passage, which other was so far subjected to the pressurization.
With this, pressurizing the operation oil of the one of the first oil passage and the second oil passage for increased oil pressure thereof cannot make the steering assistance, unless the above pressurization is over the remaining pressure of the other of the first oil passage and the second oil passage. A delay in the steering assistance may thus be caused, rapidly fluctuating torque of the steering wheel, resulting in deteriorated steering feel.